Patent GB-1325227-A (GB)
Title: Semi-permeable membranes
IPC: B01D69/12
Filed: 1971-04-19
Abstract: 1325227 Semi-permeable membranes AGENCE NATIONALE DE VALORISATION DE LA RECHERCHE 19 April 1971 [17 April 1970] 27288/71 Heading B5B [Also in Division C3] A membrane suitable for reverse osmosis comprises a layer of film-forming polymer crosslinked over at least part of its thickness in such manner that the average number of cross-links per unit area differs substantially from one face of said layer to the other. The average number of cross-links per unit area may increase uniformly from one face of said layer to the other. Thus the layer may consist of a very thin highly cross-linked superficial sub-layer and a second sub-layer which is thicker and more porous than said superficial sub-layer, or of a plurality of sub-layers, the average number of cross-links per unit area varying progressively through said sub-layers from a minimum value at one face of said layer to a maximum value at the other face of said layers. Preferably said layer has a thickness of between 0À5 microns and 10 microns. The layer may be deposited on a porous support, suitably a polymer wettable by water, the porous support being desirably in contact with the face of the layer that has the higher number of cross-links per unit area. The use of polyvinyl alcohol and polyvinyl-pyrrolidone is exemplified. The membranes are made by contacting one face only of a layer of film-forming polymer, in the dry state and in a dry atmosphere, with a cross-linking agent, suitably a di-isocyanate, especially toluylene di-isocyanate. The crosslinking agent may be used in solution, the solution desirably containing a swelling agent or a solvent for the polymer, and such solution may be used in the vapour phase. The solution may also contain a cross-linking catalyst. In a preferred method, the solvent is evaporated rapidly and/or the polymer layer is dried at a high temperature in order to accentuate the dissymmetry of structure between the sub-layer adjacent to the face of the layer contacted with the cross-linking agent, and the remainder of the layer. This is especially suitable where the solution contains a cross-linking catalyst. Alternatively, the face may be contacted with solvent and the solvent evaporated rapidly and/or the polymer layer dried at a high temperature to render the structure of the layer more dense in the vicinity of said face, before said face is contacted with the cross-linking agent. Such cross-linking may be carried out in the presence of a cross-linking catalyst, which is suitably contained in the layer of polymer. The layer of polymer may be formed by pouring polymer over a plate of glass or metal and said layer may be contacted with the cross-linking agent, while remaining fixed on the plate, or the layer may be detached from the plate and stretched over a support of glass or metal before being contacted therewith. Thus in the figure, a hollow support S of glass or metal is coated with a polymer layer F to be treated. There is thus produced a tank C 1 , the bottom of which is formed by the other polymer layer. This tank serves as a hermetic seal cover for another tank C 2  which contains reactants R used to treat the layer. The polymer layer and its support are dried before they are fixed on the tank C 2  containing the reactants, this latter operation being preferably carried out in a glove box. The assembly formed by the two tanks fixed one on the other is placed in a desiccator containing a drying agent. The desiccator is heated for several hours in a stove, the temperature of which is kept constant. Alternatively the polymer is poured over a suitably shaped form or a mould of glass or metal suitably flat or cylindrical. The mould, coated with the polymer layer, is exposed to the vaporized reactants. As previously, it is desirable that the polymer layer should be previously dried and that the treatment should be carried out in a closed chamber protected against humidity. After the cross-linking, the layer is rinsed with a solvent for the cross-linking agent, suitably benzene or ethyl acetate. The polymer layer can then be used either directly or, preferably after having been subjected to a thermal treatment in hot water, suitably at 30 to 100 C. After the crossinking step, the non-cross-linked portion of the layer may be dissolved, and the layer is then finally dried. In example 1, a layer of polyvinyl alcohol of 6 microns thickness in the dry state was prepared from a 5% by weight aqueous solution of a polyvinyl alcohol containing at least 1% of polyvinyl acetate, by pouring the solution over a plate of nickel plated brass and by evaporating water under a partial vacuum in a stove at 60 C. The layer was then stretched on a glass support, dried at 60 C. for 1 hour, exposed for 14 hours by its "nickel" face (face remaining in contact with the plate of nickel plated brass during the evaporation of water) to a vapour obtained by evaporation at 60 C. of a mixture containing 20% of dimethyl sulphoxide and 80% of toluylene di-isocyanate, rinsed with benzene, and dried at 60 C. for 15 hours. In example 15, a layer of polyvinyl pyrrolidone was prepared from a 15% solution of "Luviscol" K 90 (Trade Mark) and 0À6% solution of diamino-4-4-diphenyl-metbane in dimethyl sulphoxide by pouring the solution over a glass plate and evaporating the solvent under partial vacuum in a stove at 80 C. The layer was dried for 15 hours and then exposed for 6 hours by its "air" face to a vapour obtained by evaporation of 60 C. of a mixture of 20% by volume of dimethyl-sulphoxide and 80% of toluylene di-isocyanate. The layer was dried for 8 hours at 80C in the presence of phosphorus pentoxide. Subsequently, the layer was subjected to a thermal treatment consisting of immersion for 1 hour in water at 40 C., followed by immersion for 30 minutes in water at 85 C. Membranes so made are especially suitable for the separation from solution of solutes of low molecular weight, e.g. sodium chloride and sucrose.